The general method for producing an electrolytic capacitor comprises winding, via a separator made of Manila paper, etc., an anodic electrode foil (A), which has been obtained by chemically or electrochemically etching a valve metal foil (for example, a band made of highly pure aluminum) to enlarge the foil surface and subjecting this foil to an anodizing treatment in an electrolyte such as an aqueous ammonium borate solution to thereby form an oxide coating layer on its surface, and a cathodic electrode foil (B) made of a highly pure foil subjected to etching treatment only. Next, the obtained capacitor device is impregnated with an electrolyte for driving electrolytic capacitors and putting in a bottomed outer case. The outer case is equipped at the opening with a sealer made of an elastic rubber and sealed by drawing.
The anodic electrode foil and the cathodic electrode foil are each connected to a lead wire by stitching, ultrasonic welding, etc., so as to lead the electrode. The lead wire employed as each electrode leading means is composed of a round bar, a connecting member being in contact with the electrode foil and an outer connecting member made of a solderable metal which has been fixed at the tip of the round bar by welding, etc.
There are various electrolytes, with which capacitor devices are to be impregnated for driving electrolytic capacitors, depending on the performance of the electrolytic capacitor employed. Among them, adipic acid solutions in ethylene glycol are known as electrolytes which are suitable for low voltage and have excellent high temperature life characteristics.
FIGS. 1 and 2 show general structures of aluminum electrolytic capacitors. An aluminum foil of a high purity is chemically or electrochemically etched to thereby enlarge the aluminum foil surface. This aluminum foil is subjected to an anodizing treatment in an electrolyte such as an aqueous ammonium borate solution to give an anodic electrode foil (2) having an oxide coating layer formed on its surface. This anodic electrode foil (2) and a cathodic electrode foil (3) made of an aluminum foil of a high purity having been etched alone, are wound via a separator (11) made of Manila paper, etc., to give a capacitor device (1) as shown in FIG. 2. As FIG. 1 shows, this capacitor device (1) is impregnated with an electrolyte for driving electrolytic capacitors and then put into a bottomed outer case (10) made of aluminum, etc., which is then sealed by drawing.
As FIG. 2 shows, the anodic electrode foil (2) and the cathodic electrode foil (3) are provided respectively with lead wires (4) and (5), which are electrode-leading means, by stitching, welding, etc. The lead wires (4) and (5) each employed as an electrode leading means is composed of a round bar (6) made of aluminum, a connecting member (7) being in contact with the electrode foil (2) or (3) and an outer connecting member (8) made of a solderable metal which has been fixed at the tip of the round bar (6) by welding, etc.
There are various electrolytes, with which the capacitor device (1) is impregnated for driving aluminum electrolytic capacitors, depending on the performance of the aluminum electrolytic capacitor employed. Among them, a solution comprising a quaternary ammonium salt dissolved in .gamma.-butyrolactone is known as an electrolyte having a high electric conductance. In recent years, there are further reported electrolytes wherein .gamma.-butyrolactone is employed as the main solvent and salts composed of quaternized cyclic amidin compounds (imidazolinium cation and imidazolium cation) as the cationic component and acid conjugated bases as the anionic component are dissolved therein as the solute (JP-A-8-321440 and JP-A-8-321441. The term "JP-A" as used herein means an "unexamined published Japanese patent application").
With the recent tendency toward improved automobile functions, it has been more and more required in the field of vehicle equipment to use electronic parts in engine areas operated at high temperatures. However, electrolytic capacitors with the use of the above-mentioned electrolytes cannot withstand these high temperatures. Regarding low temperature characteristics, furthermore, these electrolytic capacitors can withstand a temperature of -25.degree. C. at lowest but vehicle equipment should generally withstand low temperatures of about -40.degree. C. Namely, no electrolyte for electrolytic capacitors usable in this field has been provided in practice so far.
Although sulfolane has been known as a high-boiling solvent capable of imparting excellent high temperature life characteristics (JP-A-1-124210 and JP-A-8-31699), it fails to establish the desired characteristics as described above. Thus, sulfolane cannot be employed in vehicle equipments which should have excellent high temperature life characteristics and low temperature properties simultaneously.